Newly developed biophysical techniques allow study of the membrane processes responsible for excitability and transduction in olfactory receptor neurons (ORNs). It appears that the cilium, which is the site of stimulus reception, has membrane properties which distinguish it from other known nervous system receptors and that it is coupled to the host cell through a highly asymmetric impedance. In frogs the membrane of the cell body is normally inactivated, apparently improving the efficiency by which the message is transmitted from the cilium to the axon. ORNs are unique in the vertebrate kingdom in that they are continually replaced throughout life, and their responses to stimulus messages change as the cells pas through the stages of their life cycle. There is evidence that newly differentiated ORNs have long dendrites. As the cells mature, cilia lengthen, dendrites shorten and responses to odorous stimuli become selective. The proposed experiments will explore the processes which mediate cell excitability at different stages of ORN growth and will determine if measures of ciliary and dendritic length are sufficient to establish the developmental state of the cell. Gigaseal patch clamp recordings from the isolated cilium, the cilium with a cell attached, and the soma (whole-cell configuration) will be used to measure how excitability changes with changes in ciliary and cell dendritic length. Fluorescent label will be injected in olfactory bulbs to retrogradely fill the most mature ORNs, those whose terminals have grown to reach the bulb. If only cells with short dendrites and long cilia are filled then dimensional measurements can be used to establish the developmental age of the cell. The olfactory sense has a major role in regulating body hormonal state, emotional disposition, hunger, and social behavior. The work proposed here will advance understanding of the molecular and cellular processes which subserve sensory function.